1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device. In particular, the present invention relates to a chemical mechanical polishing (CMP) technique to be employed in the step of isolating elements of a semiconductor device such as a system LSI, a high-speed LSI, etc.
2. Description of the Related Art
In recent years, in order to meet the trend to further enhance the fineness of semiconductor elements, various fine working techniques have been developed. Among them, the employment of the CMP technique is now considered indispensable in forming a buried flat structure.
For example, the CMP technique is employed on the occasion of forming a shallow trench isolation (STI) structure in a system LSI, a high-speed LSI, etc., wherein CMP of an oxide film is performed using a nitride film as a stopper. However, there are various problems as described below.
(1) Since it is difficult to detect the end point of the polishing, the polishing is inadvertently continued even after the nitride film employed as a stopper has been entirely exposed, thus rendering the nitride film to be excessively polished, resulting in non-uniformity in thickness of the nitride film within a single chip.
(2) Since it is difficult to detect the end point of the polishing, the polishing may be terminated prior to the moment where the nitride film employed as a stopper is entirely exposed and, due to insufficient polishing, oxide film may be left on the surface of the nitride film.
(3) Due to the influences by the fluctuation in structure of a polished surface prior to CMP, such as the thickness of a silicon oxide film and the height of steps formed on the surface of a silicon substrate, or due to the influences by the fluctuation of polishing speed of CMP, the thickness of the silicon oxide film resulting from CMP as well as the optimum polishing time are caused to fluctuate.
(4) Since scratches generated during CMP are permitted to remain even after the CMP, the properties of semiconductor elements are caused to deteriorate.
As means for solving these problems, there is conventionally known a method wherein the magnitude of electric current of the motor of the turn table of the polishing apparatus is monitored and, based on the magnitude of electric current, the end point of polishing is determined (for example, U.S. Pat. No. 6,191,037).
As another conventional means for solving these problems, there is also known a method wherein a first polishing in which it is easy to detect the end point is performed initially, thus detecting the end point, after which an additional second polishing is performed (for example, U.S. Pat. No. 6,340,434). Additionally, there is also known a method wherein a first polishing for flattening a metal layer is performed initially, after which a second polishing using a reduced quantity of a abrasive particles is performed to thereby bury a metal layer in a trench (for example, U.S. Pat. No. 5,985,748).
However, any one of these conventional methods is incapable of solving all of the aforementioned problems.